1. Field of the Invention
This invention relates to computer system input output (I/O) operations, and more particularly to an apparatus and method for improving I/O performance of a multiple I/O channel computer system at minimal cost. This invention is intended for use in computer systems controlled by a multiple tasking, multiple threaded operating system such as Windows NT, OS/2, System 7.5, Novell or UNIX, but could also provide significant cost advantages in computer systems controlled by single threaded and/or single tasking operating systems such as Windows or DOS.
2. Description of Related Art
A typical computer system generally includes one or more central processing units (CPUs), main memory and one or more I/O channels. The functions of the I/O channels are to transfer data between the CPU's main memory and either storage units or network interface devices. Storage units store data and programs which the CPU uses in performing specific programming tasks. Typical storage units include hard disk drives, CDROMs and tape drives. Network interface devices allow the transfer of data to or from other computer systems on the network. Typical network interfaces include ethernet, fiber channel, ATM and FDDI.
Recently, applications run by the CPU have migrated toward using the high-performance processing power of microprocessors (such as the Intel Pentium, PowerPC and MIPs R4400) to generate real-time, full motion video. To support these applications, substantially more I/O bandwidth is required. Typically, full motion video windows are limited in size to a fraction of the video monitor size due to limitations in I/O bandwidth and or limitations in the processing power of the CPU or video card. Users operating computer systems in multi-window, graphics intensive environments expect instant response to mouse clicks or commands. Mouse clicks on an ICON representing a large text file or application should result in immediate visual results, whether or not the selected file resides on an I/O channel, or whether or not an I/O device is currently transferring large blocks of full motion video data. This instant response expectation, when coupled with the high-bandwidth transfer requirement, dictates that future I/O controllers allow I/O requests to be preempted and rescheduled at a later time.
More recently, new techniques have been proposed for providing low-cost, high-performance I/O in the personal computer (PC) area. Enhanced IDE disk drives having large buffers, capable of interfacing to IDE channels having over sixteen megabyte per second DMA bandwidth, have been demonstrated and produced, and will likely become mainstream in the PC industry over the next few years. Brad Hosler has proposed an IDE interface specification for improving system performance in multitasking environments, called "A Programming Interface for Bus Master IDE Controllers", which assumes the capabilities inherent in the DMA capable IDE disk drives mentioned earlier. Hosler's proposal, which is hereby incorporated by reference as background material, specifies a simple scatter/gather mechanism which allows large or small blocks of I/O transfer data to be scattered to or gathered from main memory. This mechanism cuts down on the number of processor interrupts required and the total number of I/O device interactions with the CPU when transferring large blocks of data, such as that required to support full motion video. Although the scatter gather programming interface specifications were originally intended specifically for controlling hard disk drives on an IDE channel, software drivers can be written to access other types of storage devices on the IDE channel, storage devices on other types of I/O channels or network interface devices using the same homogeneous scatter gather programming interface.